Numerically controlled oscillators (or binary adder frequency synthesizers) have several desirable features which make them useful for a number of applications. Some of these features are accuracy, stability and fast locking characteristics. One drawback of utilizing a numerically controlled oscillator (NCO) in an RF generator design is the presence of spurious outputs. Generally, the digital signal output of the NCO is not a perfect square-wave (50% duty cycle) but has transitions which are erratic in time compared to an ideal output. These erratic pulses are repetitive in nature and serve to create spurious outputs.
A traditional means of reducing spurious outputs from an NCO is to utilize the digital signal output from the NCO as an address input for a sine lookup table. The sine lookup table converts the digital word from the NCO into an approximation of the amplitude of a sine wave corresponding to that value. Thus, the output is an analog sine wave rather than a digital output signal. The disadvantages of utilizing this configuration for RF signal generation are the speed limitations of the lookup table and the low frequency of the output compared to the NCO clock rate, typically 1/5 or less.
Another method of reducing spurious outputs from an NCO is to divide the NCO output by a fixed number, n. This process succeeds in reducing the spurious outputs by 20 log n, however, it also suffers the disadvantage of reducing the maximum output frequency.